Samvartta Smriti 23

23

upāsīta na cetsandhyāmagnikāryyaṃ na vā kṛtam
gāyatryaṣṭasahasrantu japetsnātvā samāhitaḥ

If one does not attend to the twilight devotion, or if the fire ritual is not performed, / having bathed, concentrated, he should mutter eight thousand Gāyatrīs.